The present invention relates to tracking heat flux sensors used to accurately control the tracking of focused solar light for concentrating solar collectors, such as point-focus (dish) and line-focus (trough) collectors; as well as concentrating photovoltaic systems. The sensors can be passively cooled and mounted adjacent to the focus of the solar collectors.
Concentrating Solar Power (CSP) systems convert the energy in concentrated sunlight into mechanical, electrical, or chemical energy. Accurate tracking of concentrating solar collectors is needed to enable higher concentration ratios and/or minimize the amount of reflected solar energy that misses the receiver. Open and closed-loop tracking are the two basic approaches for pointing the collector at the sun as it crosses the sky. In open-loop tracking, the collector is pointed at the calculated position of the sun. This requires that the accuracy of the tracking system, collector, and its physical and angular position must be equal to or better than the desired tracking accuracy. Shaft encoders on the drives and the use of inclinometers and compasses have been used to establish collector tracking position. However, systematic errors or the failure to accurately position the collector relative to the earth-sun coordinates can result in tracking errors.
Closed-loop tracking sensors typically detect the actual position of the sun and tracking is adjusted until the tracking matches the sun's position. Traditional closed-loop tracking, such as those described in U.S. Pat. Nos. 3,996,460, 4,225,781, 4,235,153, 4,672,191, and 7,109,461, use sun (light) sensors mounted on the tracking structure. The sun sensors are typically pairs of photovoltaic solar cells with one pair of sensors per tracking axis. Photo-resistors or other sensor types can also be used. Lens or shadow bands aligned with the optical axis of the collectors equally illuminate both sensors in the pair, resulting in an equal signal from the pair when properly tracking. Misalignments between the sensor and collector axis causes one of the sensors to be illuminated more than the other, resulting in an error signal that is used to correct the tracking error. Misalignment of the sun sensor, or deflections of the structure, can lead to differences between the measured and actual sun position relative to the collector/receiver axis. In addition, unequal distribution of dirt, or unequal response characteristics of the sensors can lead to tracking errors.
It is also possible to use thermal receiver temperature sensors to provide a closed-loop tracking signal. The temperature-dependence of the electrical resistance of wire has been proposed as a way to sense heat flux near the aperture of a point focus dish collector in U.S. Pat. Nos. 4,332,238 and 4,516,018. In addition, early in the U.S. Department of Energy trough development program, Sandia National Laboratories built and tested a closed-loop tracking system consisting of a pair of fine wires installed along each side and parallel to the absorber tube axis. The wires changed resistance as a function of their temperature. This approach was shown to be more accurate than a shadow-band tracker (Boultinghouse, Karlan, “Development of a Solar Flux Tracker for Parabolic Trough Collectors,” SAND82-1600, Sandia National Laboratories, Albuquerque, N. Mex., September 1982).
However, because thermocouples and heated wires measure temperature, they respond slowly to a falling flux level (e.g., when the tracking adjusts away from the sensor). The slow response can cause unstable control. They can also overheat or melt if not adequately thermally coupled to a heat sink. The competing requirements of fast temperature response vs. having adequate thermal mass to avoid overheating make accurate and responsive closed-loop tracking with thermocouples difficult. In addition, wind-driven convective heating from a thermal receiver can unevenly and unpredictably heat one sensor relative to its pair, and indeterminately affect tracking response. Controlling tracking using receiver temperatures can be susceptible to errors from non-uniform soiling, shading, frost on the collector, or engine degradation.